The present invention relates to a sheet post-processing apparatus and an image forming apparatus, and in particular, to a sheet post-processing apparatus and an image forming apparatus for forming and binding a bundle of sheets with images.
A sheet post-processing apparatus has been known for aligning ends of a plurality of sheets and forming the sheets into a bundle. The sheets are discharged from an image forming apparatus such as a printer or a facsimile machine after images are printed on the sheets. The sheet post-processing apparatus uses a stapler to staple and bind the sheet bundle (see, for example, Patent Document 1). Such a sheet post-processing apparatus discharges a bundle of sheets with the ends thereof aligned with one another after being bound. Consequently, the finished sheet bundle appears neat, and it is not necessary to manually align and bind the sheets discharged by the image forming apparatus.
Such a sheet post-processing apparatus comprises a stapler unit for stapling a sheet bundle. The stapler unit generally has a minimum number of sheets that can be bound, for example, 20 or 30 sheets. A print grade (cost) and power consumption depend on the number of sheets that can be bound using the stapler unit. In a binding apparatus, leg portions of a bind member such as a staple are penetrated into the sheet bundle at punched positions (for example, refer to Patent Documents 2 and 3).    Patent Document 1: Japanese Patent Publication (Kokai) No. 2003-267622    Patent Document 2: Japanese Patent Publication (Kokai) No. 11-20363    Patent Document 3: Japanese Patent Publication (Kokai) No. 11-20364
In the conventional sheet post-processing apparatus, it is necessary to increase a load on a staple penetrating a sheet bundle when a large number of sheets are bound. Accordingly, when the sheet bundle has a large thickness, leg portions (tip portions) of a staple may not withstand the load and may be bent, thereby causing inappropriate binding. Further, for example, color print sheets have coated surfaces and a thickness larger than normal (monochrome) sheets. Accordingly, for a certain type of sheets, the load on the staple becomes greater, thereby causing a problem similar to that associated with a large number of sheets.
To solve the problems, it is possible to increase the strength of the staple. However, in this case, a driving force greater the load is required to drive the staple into the sheet bundle. Consequently, the stapler unit and the sheet post-processing apparatus must have a large size. Noise or vibration also increases when the staple is penetrated into the sheet bundle, and power consumption increases. When a bundle of sheets is bound with a strong staple, it is difficult to remove the staple, thereby making it difficult to handle the sheet post-processing apparatus.
In view of the problems described above, an object of the present invention is to provide a sheet post-processing apparatus and an image forming apparatus capable of reliably binding a sheet bundle without increasing a size of the apparatus. It is also possible to reduce noise, vibration, and power consumption even if a large number of sheets or thick sheets are processed.
Further objects and advantages of the invention will be apparent from the following description of the invention.